Usborne books are very popular among homeschoolers, for fairly obvious reasons. Homeschoolers (in my perception) have a certain aversion to textbooks, and Usborne books nicely sidestep this problem. They also encourage further study.

Usborne books can usually be found at education-oriented children's bookstores, (Title Wave Books is the place to go in Albuquerque for them) or you can (I think) track them down through usborne.co.uk.